Abstract:

The ultimate goal of any living cell is to pass on a complete, unaltered copy of its DNA to its daughter cell. The DNA damage response (DDR) and spindle checkpoint are two essential signaling pathways that make it possible for a cell to achieve this goal. The DDR protects genetic integrity by sensing errors in the DNA sequence and activating signaling pathways to arrest the cell cycle and repair the DNA. The spindle checkpoint protects chromosomal integrity by preventing the separation of chromosomes during mitosis until all chromosomes are correctly attached to the mitotic spindle. Proper regulation of both the DDR and the spindle checkpoint is critical for cell survival. In this dissertation I will describe our discovery of novel regulatory mechanisms involved in each of these signaling networks.

In the first research chapter of this dissertation, we describe our findings concerning how the DDR regulates cyclin F levels. Cyclin F is an F-box protein that associates with the SCF E3 ubiquitin ligase complex to target proteins for degradation. In response to DNA damage, cyclin F levels are downregulated to facilitate increased dNTP production for efficient DNA repair, but the molecular mechanisms regulating this downregulation of cyclin F are largely unknown. We discovered that cyclin F downregulation by the DDR is the combined result of increased protein degradation and decreased mRNA expression. At the level of protein regulation, cyclin F is targeted for proteasomal degradation by the SCF complex. Interestingly, we found that the half-life of cyclin F protein is significantly increased in cells treated with the phosphatase inhibitor calyculin A, which caused cyclin F to be hyper-phosphorylated. Calyculin A also partially prevented cyclin F downregulation following DNA damage. This result suggests that cyclin F phosphorylation stabilizes the protein, and dephosphorylation of cyclin F may be required for its degradation in both unperturbed and DNA damaged cells. We also found that cyclin F downregulation is dependent on the Chk1 kinase, which is predominately activated by the ATR kinase. In examining the mechanism by which Chk1 promotes cyclin F downregulation, we determined that Chk1 represses cyclin F transcription. Lastly, we investigated the role of cyclin F in cell cycle regulation and discovered that both increased and decreased cyclin F expression delay mitotic entry, indicating that an optimal level of cyclin F expression is critical for proper cell cycle progression.

The second research chapter of this dissertation details our discovery of the requirement for phosphatase activity to inhibit the APC/C E3 ubiquitin ligase during the spindle checkpoint. Early in mitosis, the mitotic checkpoint complex (MCC) inactivates the APC/C until the chromosomes are properly aligned and attached to the mitotic spindle at metaphase. Once all the chromosomes are properly attached to the spindle, the MCC dissociates, and the APC/C targets cyclin B and securin for degradation so that the cell progresses into anaphase. While phosphorylation is known to drive many of the events during the checkpoint, the precise molecular mechanisms regulating spindle checkpoint maintenance and inactivation are still poorly understood. In our studies, we sought to determine the role of mitotic phosphatases during the spindle checkpoint. To address this question, we treated spindle checkpoint-arrested cells with various phosphatase inhibitors and examined their effect on the MCC and APC/C activation. Using this approach we found that two phosphatase inhibitors, calyculin A and okadaic acid (1 µM), caused MCC dissociation and APC/C activation in spindle checkpoint-arrested cells. Although the cells were able to degrade cyclin B, they did not exit mitosis as evidenced by high levels of Cdk1 substrate phosphorylation and chromosome condensation. Our results provide the first evidence that phosphatases are essential for maintenance of the MCC during operation of the spindle checkpoint.